Magnetic susceptibility of medical metals: New insights for MR conditional devices

Tue, 16 May 2023 15:13:00 Z

A recent study, coauthored by Fort Wayne Metals and MED Institute researchers, provides valuable insights into the magnetic susceptibility of 45 metallic materials commonly used in medical devices. This research offers critical data for selecting materials in applications where MR safety is a concern.

Why magnetic susceptibility matters in medical devices

Magnetic susceptibility refers to a material’s degree of magnetization in response to an external magnetic field. In the medical field, this property is essential for devices that may be used in magnetic resonance (MR) environments. Materials with high magnetic susceptibility can:

Cause image distortion in MRI scans
Generate dangerous forces and torques within strong magnetic fields
Impact the safety of implanted or external medical devices

To achieve an MR Conditional label, a device must be designed with materials that minimize these effects.

Key findings

The study measured the magnetic susceptibility of a wide range of metals and alloys, presenting results in ascending order. Key takeaways include:

Titanium and Nitinol alloys: Extremely low magnetic susceptibility, making them ideal for MR-compatible implants
Cobalt-chromium (CoCr) alloys: Moderate susceptibility but often acceptable for certain applications
Stainless steel: Vary widely depending on composition and cold working, with some grades exhibiting significantly higher susceptibility
Nickel-based alloys: Generally higher susceptibility, which can limit MR compatibility

Material selection for MR Conditional devices

For medical device manufacturers, this data supports informed material selection

Best choices for MR compatibility: Titanium and Nitinol alloys
Moderate risk materials: Certain CoCr alloys and austenitic stainless steels
High susceptibility materials to avoid: Cold-worked stainless steels and some nickel-based alloys

Conclusion

This study provides a comprehensive reference for selecting medical metals based on magnetic susceptibility. By considering this factor early in the design process, manufacturers can enhance MR safety, improve imaging quality, and ensure compliance with MR Conditional labeling standards.

Superelastic conductor materials: Enhancing implantable lead durability

Mon, 22 Aug 2022 12:00:00 Z

Implantable biostimulation leads are essential components in cardiostimulation and neurostimulation devices. These highly engineered wire constructs must endure millions of flexural cycles over decades of use. Fort Wayne Metals has introduced a new wire construct concept that significantly improves fatigue resistance, potentially enhancing the longevity and reliability of implantable medical devices.

Breakthrough in conductor materials

Traditional biostimulation leads have relied on materials like 35N LT® (CoNiCrMo), which offer good fatigue resistance but still present limitations in extreme flexural conditions. The introduction of Nitinol-based conductors presents a major advancement. When substituted for 35N LT®, Nitinol demonstrates a 50% to 100% improvement in cyclic strain-loading fatigue performance, making it a promising alternative for long-term implantable applications.

The new composite wire, NiTi-DFT®-30Ag, features a high-conductivity pure silver core encased in a Nitinol outer sheath. While its ultimate strength of 1000 MPa is lower than the 1600 MPa of traditional 35N LT®-DFT®-28Ag, it requires significantly more energy to fracture—65.9 mJ/mm³ compared to 23.7 mJ/mm³. This enhanced toughness suggests improved resilience under continuous flexing conditions.

Advantages for medical device applications

Nitinol’s unique superelastic properties allow it to elastically recover from strains exceeding 10%, making it an excellent candidate for applications requiring high fatigue resistance. This property has already led to its widespread use in guidewires and stents, and its introduction into implantable leads could revolutionize the field.

A key challenge in integrating Nitinol into lead designs has been its elasticity, which complicates coil formation and shape retention. However, Fort Wayne Metals has successfully demonstrated that polyimide coatings can provide the necessary electrical insulation while withstanding high-temperature shape-setting processes. This breakthrough enables the use of Nitinol-based bifilar coils, which maintain electrical isolation even after exposure to 450-550°C secondary shape-setting.

Potential future applications

The ability to shape set Nitinol-based conductors at high temperatures opens up new design possibilities for implantable leads. For example, future pacing and defibrillation systems may incorporate hybrid designs with transmyocardial leads that require exceptional flexibility and durability. Additionally, Nitinol-based leads could be programmed with deployable shapes that enhance passive fixation, reducing the risk of dislodgement or migration.

Beyond cardiac applications, neurostimulation devices could benefit from Nitinol’s compliance and resistance to mechanical fatigue. As neurostimulation leads are often subjected to constant movement within the body, improved fatigue resistance could lead to longer-lasting and more reliable therapies for conditions such as Parkinson’s disease and chronic pain.

Conclusion

The introduction of Nitinol-based conductors represents a significant advancement in implantable lead technology. With superior fatigue resistance, enhanced durability, and the ability to maintain electrical integrity under extreme conditions, these materials have the potential to redefine the future of biostimulation devices. As research and development continue, Nitinol-based leads may soon become the new standard for next-generation implantable medical devices.

Fort Wayne Metals - Technical Blog

Magnetic susceptibility of medical metals: New insights for MR conditional devices

Why magnetic susceptibility matters in medical devices

Key findings

Conclusion

Superelastic conductor materials: Enhancing implantable lead durability